1. Field of the Invention
The present invention relates to a valve operating device for operating a valve such as an intake valve or an exhaust valve in an internal combustion engine.
2. Prior Art
One conventional valve operating device for use in an internal combustion engine includes a camshaft having a cam for alternately opening and closing an engine valve such as an intake valve or an exhaust valve in the engine, the engine valve being held against one end of a cam follower or rocker arm, the other end of which engages a hydraulic lash adjuster. The cam has a cam profile composed of a cam lobe and a base circle portion. The cam has on its cam profile a valve opening point where the rocker arm contacting the cam opens the valve and a valve closing point where the rocker arm contacting the cam closes the engine valve. The base circle portion includes a gradient cam surface sloping progressively downwardly toward the circumference of the base circle or radially inwardly with respect to the cam, in a circumferential direction from the valve closing point toward the valve opening point for preventing the engine valve from suffering a valve closing failure due to cam vibration resulting from undesirable radial displacement or flexure of the camshaft. The radial distance between the valve opening and closing points is selected to correspond to, or be slightly smaller than, a play or lift loss in the hydraulic lash adjuster for allowing certain unwanted radial valve-lifting displacement of the base circle portion to be canceled out or offset by the radially inwardly sloping gradient cam surface of the base circle portion, without varying the timing to open the valve. Such a valve operating device is disclosed in U.S. Pat. No. 4,538,559, for example. The disclosed hydraulic lash adjuster includes a check valve in the form of a ball normally biased in a closing direction by a spring. Any play or lift loss in the hydraulic lash adjuster is therefore limited to the amount of resilient depression of its plunger on account of compressive deformation of air bubbles in the oil in the lash adjuster at the time the lash adjuster is under load, and the amount of depression of the plunger due to hydraulic pressure leakage therefrom while the engine valve is being closed.
The amount of resilient depression and the amount of leakage-dependent depression of the plunger of the lash adjuster generally range from 20 to 30 .mu.m. Therefore, the radial distance between the valve closing and opening points on the cam profile is also in the range of from 20 to 30 .mu.m at most insofar as the timing to open the engine valve is not varied. However, the base circle portion of the cam is often subject to radial valve-lifting displacements beyond the above numerical range due to machining errors, flexure, or the like, and hence such radial valve-lifting displacements cannot be offset by the radially inward gradient on the base circle portion.
One solution would be to increase the amount of depression of the plunger of the lash adjuster due to hydraulic pressure leakage from the plunger, thereby increasing the radially inward gradient on the base circle portion. However, such a scheme would result in a reduction in the maximum opening the engine valve can provide for supplying an air-fuel mixture into the combustion chamber, so that the output power of the engine would be lowered.
The inventors have found that large radial valve-lifting displacement of the base circle portion of the cam tends to occur in a localized region, particularly, immediately after the engine valve has been closed, rather than throughout the entire cam profile between the valve closing and opening points. It has also been found that where the internal combustion engine has a plurality of engine valves of one type on a common camshaft, the base circle portions of the cams are liable to undergo different valve-lifting displacements dependent on the positions of the cams. If such localized or different valve-lifting displacements are to be canceled out by the conventional valve operating device, the play in the hydraulic lash adjuster has to be increased and so does the radially inward gradient on the base circle portion between the valve closing and opening points. The increased play in the hydraulic lash adjuster, however, modifies the opening characteristics or pattern of the engine valve, i.e., delays the opening timing of all engine valves and reduces the opening strokes of the valves.